It might be harder than usual to make friends when you're underslept. Or, at least, that's my somewhat hammy interpretation of a recent Swedish study in which sleep loss corresponded to people becoming less socially desirable. Researchers from Karolinska Institute in Stockholm also found that, after two nights of restricted sleep, people were perceived as looking less healthy, less attractive and, perhaps unsurprisingly, more tired than they did when they were well-rested.

Why would the fact that someone is low on sleep cause their social stock to plummet? Well, for one thing, previous research suggests that sleep-deprived faces are perceived as less attractive than well-rested ones. We humans have a tendency to, as researchers wrote, "ascribe positive qualities, especially qualities related to social competence, to people who are considered attractive." This is the third study (and the second from the Karolinska team) to associate sleep loss with perceptions of reduced attractiveness.

There may also be evolutionary reasons the underslept among us haven't nailed down weekend plans. Poor sleep is associated with all sorts of health issues. "Humans, like many other animals, tend to be disease avoidant," researchers wrote. "Having an unhealthy looking face ... might thus activate disease-avoidance mechanisms in others and render one’s surroundings less socially inclined."

Not to mention, exhausted people notoriously struggle with social interactions — research suggests that reading facial expressions and picking up on sarcasm gets harder when you don't sleep. So, for the sake of putting ourselves in enjoyable social situations, we learn to leave yawning co-workers off happy-hour Slack chats.

In the current study, researchers sought to look at the relationship between social desirability and sleep restriction. In real life, it's more common to be underslept than drastically sleep-deprived. So, while past studies have assessed how perceptions of people change after they've been up for two days straight, the Karolinska researchers wanted to explore the effects of more realistic levels of sleep loss.

To do this, they set up an experiment: First, they took two sets of photos of 25 Swedish college students. The first set was taken after the students had undergone two nights of partial sleep restriction (about four hours of sleep per night). The second set was taken two days later, after the students had gotten normal sleep. Aside from the different sleep conditions, the two photo sessions took place under identical circumstances.

Researchers then asked a different group of participants to view the photos, one by one, and rate their willingness to socialize with the subjects (the students) depicted in them. The raters also had to indicate how attractive, healthy, trustworthy and sleepy the photo subjects seemed.

The raters didn't know anything about the photo subjects or their sleep habits. But, as researchers predicted, the raters were less inclined to socialize with the "underslept" photo subjects, who were also deemed less attractive, less healthy and sleepier than the well-rested subjects. Did the raters have less interest in hanging out with the sleep-restricted subjects because they looked ugly, ill and tired? To some degree, probably. But researchers' analysis suggested that something else about their faces — meaning a quality not measured in the study — lessened raters' desire to befriend them. But researchers don't know what that quality is.

In addition to establishing a link between sleep restriction and decreased social desirability, researchers wrote, the study "adds to previous studies on facial appearance after sleep loss, showing that despite using a different scale, and a less substantial and more natural sleep-loss condition, the relationships between sleep, attractiveness and a healthy appearance still hold."

But it's still a small experiment on white (mostly young) adults that leaves a lot of questions unanswered. In addition to studying a more diverse group of people, future similar studies could:

Use photos of sleep-restricted friends or family members rather than strangers

Ask raters about their willingness to work with, rather than socialize with, sleep-restricted people

Study living, yawning people in real social situations, rather than still photos of strange faces

And here's one more question that we can't answer yet: Does decreased willingness to socialize with sleep-restricted people lead to "overt ostracism"?

I'd investigate the question myself, but I'm pretty spent and all my friends seem to have plans tonight...